Trivial: Use `createTimer` for Client gossip timer

[mkykadir]

It is immediately stopped after setting, createTimer is more suitable for those conditions.

[codecov[bot]]

Codecov Report

Merging #3306 (82c5e2c) into v0.x.x (be9b8d1) will increase coverage by 0.70%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           v0.x.x    #3306      +/-   ##
==========================================
+ Coverage   87.83%   88.54%   +0.70%     
==========================================
  Files         164      165       +1     
  Lines       16964    17073     +109     
==========================================
+ Hits        14901    15117     +216     
+ Misses       2063     1956     -107     

Flag	Coverage Δ
integration	34.71% <100.00%> (?)
unittests	87.57% <100.00%> (-0.27%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
source/agora/network/Client.d	85.59% <100.00%> (-2.65%)	:arrow_down:
source/agora/common/Task.d	88.88% <0.00%> (-11.12%)	:arrow_down:
source/agora/consensus/BlockStorage.d	68.90% <0.00%> (-8.49%)	:arrow_down:
source/agora/consensus/protocol/Nominator.d	90.58% <0.00%> (-1.07%)	:arrow_down:
source/agora/node/Validator.d	90.95% <0.00%> (-0.97%)	:arrow_down:
source/agora/consensus/state/Ledger.d	88.85% <0.00%> (-0.66%)	:arrow_down:
source/agora/flash/Node.d	80.67% <0.00%> (-0.43%)	:arrow_down:
source/agora/consensus/Ledger.d	97.41% <0.00%> (-0.15%)	:arrow_down:
source/agora/network/Manager.d	77.21% <0.00%> (ø)
... and 10 more

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